Certain receiver designs have been developed to receive and demodulate a binary data signal which is modulated on a radio frequency carrier by frequency shift keying ("FSK") modulation. One such prior art receiver is illustrated in FIG. 1. In the prior art receiver, the frequency of a local oscillator 100 is set equal to the received carrier frequency. A radio frequency carrier, FSK modulated with a data signal, is picked up by an antenna 102 and mixed with the local oscillator output in mixers 104 and 106. The output of mixer 106 is an in-phase baseband signal, however, the output of mixer 104 is a quadrature phase baseband signal because the local oscillator output is phase shifted 90 degrees by a phase shift network 108 before being injected into mixer 104. The in-phase and quadrature phase baseband signals are filtered respectively by low pass filters 110 and 112. After limiting, binary in-phase and binary quadrature phase baseband signals appear respectively at the outputs of limiters 114 and 116. Each binary baseband signal is then clocked on both leading and trailing edges of the other binary baseband signal by flip flops 118, 120, 124, and 126. NOR gates 130, 132, 134, and 136 form a combinational logic circuit that is used to set and reset S-R latches 138 and 140 based on particular combinations of the flip flop output states. The outputs of S-R latches 138 and 140 are then combined by resistors 142 and 146 to produce an output signal at output 144. If both outputs of S-R latches are high, a logical 1 is indicated at output 144. If both outputs of S-R latches 138 and 140 are low, a logical 0 is indicated at output 144. If one S-R latch output is high and the other output is low, output 144 is in some undefined state.
One disadvantage of the prior art circuit is that a half amplitude error can occur at output 144 when a weak signal is received and noise is introduced into the receiver. A slight frequency offset between the transmitter frequency and the frequency of receiver local oscillator 106 can compound the problem. Typical transmitter and local oscillator stabilities are respectively +/-5 and +/-15 ppm. This results in a transmitter/local oscillator frequency offset of +/-20 ppm which translates to an absolute offset of +/-3 KHz for a 150 MHz transmitter carrier frequency. Measurements indicate that at 4 KHz deviation and at a transmitter/local oscillator offset of 2.5 KHz, the performance of the prior art circuit degrades by 8 dB from its performance at zero offset. When the deviation is reduced to 3 KHz, the prior art circuit is incapable of detecting data at 2.5 KHz offset.
Another disadvantage of the prior art circuit is found in phase shift network 108. Phase shift networks typically include coils which are well known radiators of spurious electromagnetic radiation. To prevent such spurious radiation, it is advantageous to keep the signal strength at the input of such a device as low as possible. In the prior art circuit, however, phase shift network 108 is connected to the output of local oscillator 106 which generates a very strong signal, typically -10 dBM. If some of the local oscillator's signal is radiated, other receivers in close proximity to the prior art circuit can be desensitized.